This invention relates to a device for use in processing a baseband input signal and a carrier signal into an envelope output signal and, more particularly, to an improvement in spectrum suppression under a burst switching condition used in a TDMA type radio communication apparatus in which a modulated signal has an envelope.
In general, a device of the type described is supplied with a baseband input signal and a carrier signal to produce an envelope output signal.
The device comprises a modulating unit responsive to the baseband input signal and the carrier signal for producing a modulated signal variable within a power range. A waveform generating unit is supplied with a burst control signal and generates a control signal for controlling the power range into a controlled region. An amplifying unit is connected to the modulating unit and the waveform generating unit and amplifies the modulated signal having the controlled region to make the envelope output signal represent an envelope of the modulated signal and to produce the envelope output signal.
A conventional device is disclosed in U.S. Pat. No. 4,592,073 issued to Kyoji Watanabe and assigned to NEC Corporation. According to Watanabe patent, the device comprises a waveform generating unit for receiving the burst control signal having a rectangular waveform and for generating the control signal for controlling the power range into the controlled region. The control signal has a controlled shape comprising sloping leading and trailing edges and a plateau between the sloping leading and trailing edges. The modulating unit always modulates the baseband input signal and the carrier signal into the modulating signal with no consideration given to the burst control signal. The amplifying unit amplifies the modulating signal having the controlled region to make the envelope output signal represent the envelope of the modulating signal. As a result, the envelope output signal has a shape which is similar to the controlled shape. Therefore, the modulated signal is amplified throughout the sloping leading and trailing edges and the plateau.
However, it has now been found by the present inventor that it is impossible to suppress a spectrum divergence of the envelope output signal within a predetermined guard mask which is a permitted limit of the spectrum divergence in the envelope output signal. When the spectrum divergence overrun the guard mask, a jamming occurs as a result of the envelope output signal. Therefore, a transmission interference occurs. This is because the modulated signal inevitably has an inherent band. Even if the amplifying unit tries to amplify the modulated signal in due consideration of the inherent band in order to avoid such a trouble, a very delicate adjustment is required in the amplifying unit.